Dual personality Scientists have unravelled the mystery of why some meteors flash across the night sky burning up as shooting stars, while others survive raining gently down to the ground.

The discovery by Dr David Nesvorny from the Southwest Research Institute in Boulder, Colorado and colleagues was made during a study of an astronomcical feature known as the Zodiacal Cloud.

The Zodiacal Cloud is a diffuse glow of scattered sunlight in the night sky. Models predict that micro-meteoroids enter the Earth's atmosphere at relatively low speeds, gently raining down onto the ground. But meteor radars consistently see the sky filled with rapidly speeding meteors flying too fast to survive.

Nesvorny and colleagues found the inconsistency was caused by some meteor radars not being able to see the slower, smaller meteors, instead only detecting the bigger faster ones which people see as shooting stars.

Reporting in the Astrophysical journal and on the pre-press website ArXiv.org, Nesvorny and colleagues found that 90 per cent of the dust in the Zodiacal Cloud comes from Jupiter family comets such as 2P/Encke, whose orbits are influenced by the giant gas planet.

These appear to have compositions which cause them to slow down rapidly once they enter the Earth's atmosphere, allowing them to survive and reach the ground often as micrometeorites no bigger than grains of dust.

The remaining 10 per cent of material in the Zodiacal Cloud comes from Oort Cloud comets, Halley-type comets and asteroid collisions.

They are usually associated with cometary debris trails seen as meteor showers, and because they're travelling faster when they hit the Earth's atmosphere, they're more likely to burn up as shooting stars.

Meteoroids and their tiny counter parts micro-meteoroids often fly through space at speeds of 40,000 kilometres per hour. Meteors that enter the Earth's atmosphere and reach the ground are called meteorites.

A new view

Astronomer Dr Fred Watson from the Australian Astronomical Observatory says the study provides a new insight into the amount of dust and debris falling into the Earth's atmosphere every year.

"Their figure of 15,000 tonnes a year is considerably more than previously thought," says Watson.

"When you think about it, that's an extraordinary amount of stuff falling down to the Earth's surface. It's a train's worth of stuff."

Watson says the study also provides an important contribution to our understanding of the Earth's environment," says Watson.

"It's an area of study that hasn't been examined over the past couple of decades, and we're now starting to see real scientific measurements being brought to the process rather than just the guesstimates."